Technology: Genetic gun makes rice growers' day

A genetic gun which shoots foreign genes into plant cells may soon provide
farmers in developing countries with cheap, genetically engineered rice
with built-in resistance to pests that devastate their plantations. Scientists
in Wisconsin have used this technique for the first time to alter the genetic
constitution of Indica rice varieties. These account for four-fifths of
global rice production and provide the staple food for at least 2 billion
people.

The scientists at Wisconsin are negotiating with the Rockerfeller Foundation
to work out a way to provide the varieties cheaply – perhaps even free –
to the countries that need them most. They hope to make money by selling
transformed commercial varieties in the US and Japan.

In the future, they hope to make Indica varieties resistant to widespread
pests. These include tungro virus – common in Southeast Asia, India and
China – which completely destroys crops. Another candidate is a beetle called
the rice water weaver, which damages yields by burrowing into and colonising
the roots.

Paul Christou and colleagues at Agracetus, a company based at Middleton
in Wisconsin, report in Bio/Technology (October, p 957) that genetic engineers
have already perfected methods to transform commercial Japonica varieties
of rice grown in the US and Japan. These involve suspending ‘naked’ embryonic
rice cells, which have been stripped of their cell walls, in solution. Researchers
introduce foreign DNA into these ‘protoplasts’ through open pores in the
cell surfaces. They make the pores open up either with an electric field,
by a process known as electroporation, or with the chemical polyethylene
glycol.

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Indica rice proved more stubborn than Japonica, and researchers found
it impossible to grow new Indica rice plants from material treated in this
way. Christou and colleagues tried a new approach that had already worked
in 1988 with soya.

The process, known as electric discharge particle acceleration or ‘biolistics’,
involves firing the foreign DNA like a bullet into immature embryos that
still retain their outer skin, or scutellum. The technique is a refinement
of a system pioneered by John Sanford of Cornell University (see ‘Brave
new botany’, New Scientist, 3 June 1989).

In essence, Christou bombards the rice cells with tiny gold beads coated
with plasmids, loops of DNA containing the foreign gene to be transplanted.
He first lays all the beads on a piece of aluminium foil about a centimetre
square.

A ‘gun’ points upwards at the underside of the foil. The nozzle of the
gun is 13 milli-metres across and contains a water droplet suspended between
two tiny electrodes. When Christou applies a certain voltage across the
10-millilitre droplet, it vaporises with such force that the shock wave
propels the overlying foil upwards.

A mesh above the aluminium blocks the foil but not the accelerating
beads of gold which continue upwards to reach the rice cells, imbedded in
an upturned plate of culture medium.

Christou proved that he had transplanted foreign genetic material by
inserting genes conferring resistance to certain antibiotics and herbicides.
The modified plants were the only ones that survived exposure to these toxic
compounds; normal plants died.